Displays and electronic devices have evolved to be curved, bent, or folded and provide new user experiences. These device architectures may include flexible organic light emitting diodes (OLEDs), plastic liquid crystal displays (LCDs) and the like, for example.
In order to realize such flexible displays and protect elements in the displays, a flexible cover sheet or flexible window film replaces a conventional glass cover sheet. This flexible cover sheet has a number of design parameters such as; high visible light transmission, low haze, excellent scratch resistance and puncture resistance, in order to protect the elements included in the display devices. In some cases the flexible cover sheet may also need to withstand thousands of folding events around a tight bend radius (about 5 mm or less) without showing visible damage. In other cases, the flexible cover sheet must be able to unfold without leaving a crease after being bent at elevated temperature and humidity.
A variety of hard coated plastic substrates have been explored. More exotic materials like hard coated colorless transparent polyimide films have also been shown to have high hardness and good scratch resistance. However many hard coated films fail to withstand folding events around a tight bend radius without showing visible damage.